Method of inhibiting corrosion of iron and steel



3,050,360 Patented Aug. 21, 1962 3,050,360 METHOD OF HJHIBITINGCORROSION F IRUN AND STEEL Huib de Bees and Willem F. Jense, Amsterdam,Netherlands, assignors to Shell Oil Company, New York, N.Y., acorporation of Delaware No Drawing. Filed Sept. 19, 1960, Ser. No.56,669 Claims priority, application Netherlands Nov. 26, 1959 1 Claim.(Cl. 21-2..7)

The invention relates to the inhibition that is to say the prevention orreduction, of the corrosion of iron and steel, especially the corrosionof iron and steel which comes into contact either continuously orintermittently with water containing chlorine ions. In thisspecification the term iron and steel is to be understood to includealloys of iron. The invention is particularly important with respect tothe metal surface of cargo tanks in marinetankers.

The walls of the tanks are almost continually exposed to corrosiveinfluences. Petroleum or petroleum products transported by these tankersmay cause corrosion on account of the great solubility of oxygen inthese products and the small amount of water which is usually present insuch a cargo.

Serious corrosion is also caused by the periodical cleaning of the tankswhich, in order to save time, is usually done at sea and consequentlywith sea-water. For this purpose the sea-water is often heated to 80 C.to 90 C. and then sprayed until pressure against the walls in order toremove as much rust as possible together with the oil residues. Finally,when the tank is empty the walls are corroded by the atmosphere,especially at sea.

In view of the losses sustained by contamination of the productstransported, there is a more urgent need for effective methods ofinhibiting corrosion in tankers than under other conditions.

When treating the Walls of the cargo tanks with corrosion-inhibitingmaterials, known as inhibitors, great care should be exercised in thechoiceof the inhibitor. These materials cause turbidity or haze in theoil products transported even when present in small concentrations, asthey promote the emulsification of water in the oil phase. Otherinhibitors are soluble in oil and this is also undesirable. Moreover, ithas been found that the effect of many proposed inhibitors isinsufficient for the present purpose.

In the co-pending United States patent application No. 757,111, a methodis described of inhibiting the corrosion of the walls of cargo tanks intankers in which the walls are contacted with an aqueous solution ofpotassium ferrocyanide (11 to 24% by weight). This process issatisfactory, but it has been found possible to obtain a still betterprotection against corrosion by using a compounded inhibitor mixture.

It has now been found that the said improvement is obtained by treatingthe iron and steel with an aqueous solution of alkali metal nitrite orcalcium nitrite, alkali metal acetate and an alkanolamine, the solutioncontaining not more than 89% of water as well as 3-7 parts by weight ofalkali metal nitrite or calcium nitrite to 2-6 parts by weight of alkalimetal acetate to 2-5 parts by weight of alkanolamine.

The maximum effect is obtained by means of a solution containing 4-6parts by weight of alkali metal nitrite or calcium nitrite to 3-5" partsby weight of alkali metal acetate to 34 parts by weight of alkanolamine.Outside the extreme limits specified there is a marked reduction in thecorrosion-inhibiting effect.

The preferred alkali metal nitrite used is sodium nitrite and thepreferred alkali metal acetate is sodium acetate, while the prefererdalkanolamine is trior di-ethanolamine or a mixture of water-solublealkanolamines. A greater dilution of the solution than that specifiedabove as the extreme limit, viz. with more than 89% of water, gives areduced or inadequate protection, at least if the iron or steel has notbeen previously treated; concentrated solutions hardly lead to anyimprovement in the result. With the second or subsequent treatment ofthe iron or steel it is, however, possible to maintain the protectiveeifect by treating the surface of the metal from time to time with thesolution of a much lower concentration which does not, however,preferably have a water content exceeding The surfaces to be protectedmay be treated with the above solutions according to any technique knownin the art. Fairly large surfaces, for example, the walls of the cargotanks in tankers may be sprayed with the solution. This treatment may becarried out at normal temperature, i.e. the ambient temperature, but ifdesired, also at elevated temperature, for example, 40 C.50 C. No othersubstances, for example, various phosphates, which are found ininhibitor solutions need be incorporated in the solution. The presenceof chlorine ions in the solution does not neutralize thecorrosion-inhibiting effect.

It has been found that excellent results can be obtained by using aconcentrate solution containing 23 parts by weight of sodium nitrite, 17parts by weight of sodium acetate and 14 parts by weight oftri-ethanolamine and 46 parts by weight of water, and reducing theconcentrations by dilution with amounts of water to produce a treatingsolution containing 3-7 parts by weight of sodium nitrite, 2-6 parts byweight of sodium acetate and 2-5 parts by weight of tri-ethanolamine.

EXAMPLE I Steel strips containing 0.1% of carbon were treated in thefollowing way.

The sand-blasted strips were immersed in synthetic sea-water for fourhalf-hourly intervals spread over one day during two days. They werethen suspended above synthetic sea-water for two days and then treatedin the following way:

1st day: sprayed once with synthetic sea-water until they were justwetted;

2nd day: sprayed once with the inhibitor solution until just wetted;

15th day: immersed for one hour in kerosine in which 0.1 of syntheticsea-water had emulsified;

29th day: as for 1st day, etc.

Rate of corrosion Concentration of active material (reduction in(percent by weight) thickness) (mm. per annum) 3 The synthetic sea-waterwas prepared by mixing equal volumes of the following solutions I and IIand by adjusting the pH values to 8 by adding a solution of 0.1 N NaHCOSolution I NaCl 24.5 KCl 0.69 KBr 0.10 Na]? 0.003 N21 SO 4.09 NaHCO 0.20H BO 0.03

Solution II MgCl .6H O 11.10 CaCl 1.16 SIC12-6H20 0.04

EXAMPLE II Only the treatment schedule was modified, other details beingthe same as those of the first example.

In this case the treatment schedule is as follows:

1st day: sprayed once with synthetic sea-water until just wetted;

2nd day: sprayed once with a solution containing 13.5%

of active material until just wetted;

15th day: immersed for one hour in kerosine in which 0.1% by weight ofsynthetic sea-water had emulsified;

29th day: sprayed once with synthetic sea-water until just wetted, etc.

The cycle was repeated four times, i.e. to the 141st day but during thecycles succeeding the first cycle a solution was used which contained5.4% of active material.

After 141 days the rate of corrosion was found to be 0.04 mm. per annum.

The method according to the invention has a number of considerableadvantages over the methods of treatment of iron and steel withcorrosion-inhibiting materials which have hitherto been known. Thus thepresent inhibitor is comparatively inexpensive and owing to the goodsolubility concentrated. solutions may be prepared which is advantageousin connection with transport and storage. Moreover, the solubility atlow temperature is excellent so that there is scarcely any risk ofcrystallization. In addition there is no hazing of the oil productswhich have been in contact with the inhibitor solution. The solubilityin hydrocarbons is also extremely low. Finally, the inhibitor has notbeen found to have any effect on the fuel properties of the fueltransported in treated cargo tanks.

We claim as our invention:

A corrosion inhibiting process for ferrous metal cargo compartments of asea-going oil tanker which comprises: coating the metal inner surface ofsaid compartment containing residual amounts of refined petroleumproducts with an aqueous concentrate containing 3-7 parts by weight ofsodium nitrite, 2-6 parts by weight sodium acetate and 2-5 parts byweight of tri-ethanolamine and the balance to make parts by weight beingwater, said concentrate being diluted with water to give a total activematerial concentration in final dilution of in excess of about 5.4% byweight and said final aqueous composition being coated on said metalsurface.

References Cited in the file of this patent UNITED STATES PATENTSWighton et al. Nov. 21, 1939 OTHER REFERENCES

